Small electronic components such as integrated circuits or "chips" are often carried in linear packages known as "tapes." These tapes often consist of plastic strips of molded pockets, and are frequently bordered by sprocket holes. By packaging the components in such a manner, the strips can be mechanically fed or advanced along a fully or partially automated assembly line. In such lines, the components are frequently removed from the strip's pockets by automated (e.g. robotic) equipment which then places the components in their next desired position, for example on a circuit board. When originally packaged and shipped in the strips, the pocketed components are also secured in place by a cover that overlies the pockets.
The overlying cover often takes the form of an adhesively backed tape that matches the width of the pocketed strip. Preferably, the tape should be transparent so that the identity of the components in the pockets is clearly visible. Furthermore, in many circumstances the tape also should be static dissipative or antistatic in character in order to protect the components from the potentially harmful effects of static electricity. In addition to negatively affecting the packaged components, static electricity can cause handling problems by attracting dust, or even by attracting lighter weight components to the tape in a manner analogous to the way in which a child's plastic balloon can be made to stick to a wall.
As generally used, the terms "antistatic" and "static dissipating" both refer to the same property: conductivity. The term "static dissipating" generally refers to a higher conductivity than does the term "antistatic." For example, antistatic is often used to characterize resistivities of 10.sup.9 to 10.sup.14 ohms per square, while static dissipative is used to characterize resistivities of 10.sup.5 to 10.sup.9 ohms per square. It will be understood that these terms are thus used descriptively rather than in any absolute or unreasonably limiting sense. In the invention set forth in the Detailed Description that follows, the structures are of higher conductivity and thus best described as static dissipative.
Some of the traditional or conventional compounds used to obtain antistatic and static dissipative properties are carbon black and the quaternary ammonium salts. Carbon black, however, lacks transparency when used in amounts sufficient to provide the desired static dissipative properties. The quaternary salts are disadvantageously sensitive to moisture so that their conductivity can change based on changes as slight as changes in the ambient relative humidity. Therefore, a static dissipative cover tape for integrated circuit components or the like needs to be more transparent than carbon black and more consistent in its antistatic properties than quaternary ammonium salts.
Furthermore, because automated or robotic equipment is often used to open the packages and remove the cover tape, the tape must exhibit very consistent adhesive characteristics. Stated differently, the force needed to remove the tape must be consistent within each package and from package to package. Such adhesive characteristics are also referred to as "controlled release."
Finally, because static electricity can build up on either or both sides of the cover tape, both sides should have antistatic characteristics. The presence of adhesive on one side, however, can interfere with any desired static dissipative coating.